We are using the bacteriophage T4 rII system as a model to explore mechanisms of DNA damage and mutagenesis. Because traditional DNA sequencing methods for analyzing the molecular nature of rII mutations are laborious and slow, we are developing methods based on genomic sequencing and/or the polymerase-chain-reaction method of sequence amplification. Then, important classes of mutations will be examined for their sequence changes. For instance, even mild heat damages DNA and could, if uncountered by repair, produce on the order of 100 mutations per diploid human cell per day. Earlier studies showed that heat induces both transitions and transversions at G:C base pairs in phage T4. Genetic studies suggested that the main heat-induced transversion pathway is G:C leads to C:G but did not exclude G:C leads to A:T, and the distinction can be easily resolved by sequencing studies.